1. Field of the Invention
The present invention relates to improvements in or relating to direct modulation FM data receivers in which a local oscillator frequency is located between two signalling frequencies thereby deliberately folding the transmitted spectrum about d.c. More particularly the present invention relates to automatic frequency control in such receivers which may be used in receiving and demodulating frequency shift keyed (F.S.K) signals as may be used in digital paging in which the bit rate, for example 512 bits/second is lower than the deviation (.DELTA.f), for example 4.5 kHz.
A direct modulation FM data receiver in which a local oscillator frequency is located between two signalling frequencies is disclosed in British Patent Specification No. 2109201A, to which U.S. Pat. No. 4,523,324 corresponds, details of which are incorporated herein by way of reference. This known receiver comprises a mixer having a first input for receiving a directly modulated FM signal with a given deviation (.DELTA.f), and a second input for a local oscillator signal having a frequency within the signal channel but offset from the input signal carrier frequency by an amount or offset (.delta.f) which is normally selected to be less than the deviation; and a demodulator for distinguishing between the upper and lower signalling tones (.DELTA.f+.delta.f) and (.DELTA.f-.delta.f) which are generated in the mixer, and deriving an output data signal therefrom.
An important consideration in such receivers which for convenience will be referred to as "offset receivers" is frequency stability of the carrier frequency and the local oscillator frequency. Ideally there should be a constant frequency difference (offset) between them in order to maintain a constant frequency spacing between the low and high frequency tones. For example with a deviation or center frequency of 4.5 kHz and an offset of 2.25 kHz then the lower frequency tone will be 2.25 kHz and the higher frequency tone will be 6.75 kHz. If the local oscillator frequency should drift further from the transmitter carrier by, for example, 2 kHz then the effect will be to increase the frequency separation between the tones so that their respective frequencies are 250 Hz and 8.75 kHz. Consequently the frequency discriminator would have to work to the ends of a range from d.c. to 9 kHz which is not desirable because the performance of the discriminator drops off at tone frequencies below the bit rate.
Alternatively if the local oscillator frequency should drift closer to the carrier frequency then the frequency separation between the tones diminishes, making reliable demodulation of the tones more difficult. When the carrier and local oscillator frequencies are the same then other receiver architectures have to be used.
Mistuning of up to 2 kHz in either direction is possible because local oscillator frequency drifts of up to 1 kHz at VHF can be expected; and also in some transmission systems transmitter offsets of up to 1 kHz are introduced.
2. Description of the Prior Art
Accordingly there is a need for an automatic frequency control system in such receivers. British Patent Specification No. 2109201A discloses two alternative AFC systems. Each has its own disadvantages. In the case of the simpler AFC system in which a low pass filter is coupled between the receiver output and a control input of a local oscillator, it was found that the trimming of two bandpass filters used in the demodulating means was critical to avoid spurious locking positions. The second AFC system required providing another mixer and local oscillator coupled to the output of the first mentioned mixer, as well as a frequency discriminator whose output is coupled to a control input of the local oscillator. Although this second AFC system has been shown to work it is nevertheless somewhat complicated.
An AFC system for an FM receiver is also known from British Patent Specification No. 2059702A. However, unlike the offset receiver to which the present invention relates, the AFC system tunes a local oscillator whose frequency remains to one side of the transmitted signal; that is, .delta.f&gt;.DELTA.f. In this known receiver a conventional discriminator can be used to generate the appropriate AFC transfer function which is linear. By increasing the loop gain and lengthening the time-constant of the AFC loop, to prevent the local oscillator following the modulation, a predetermined offset can be maintained between the local oscillator and the transmitted carrier. This can only be achieved as long as the transmitted signal remains on the correct side of the local oscillator, the nominal offset from which must be greater than the deviation. Such an arrangement cannot be used when the offset is smaller than the deviation because a non-linear discriminator transfer characteristic must then be used. The realization of such a non-linear discriminator transfer characteristic cannot be done very simply.